In electronic devices, portable and otherwise, it is standard to provide a real time clock (RTC). This is an electronic unit which keeps track of the date and time of day, which information is available for the other components of the device. Virtually all RTCs use a crystal oscillator, the frequency of which is usually 32.768 kHz. In current electronic devices, RTCs and associated components are situated in a power management integrated circuit (PMIC) of the device.
To function correctly, i.e. to maintain an accurate track of the date and time, the RTCs and associated components must be continuously powered. When the electronic device in which an RTC is situated is powered (e.g. from an external power source or a battery), the RTC and associated components use this power source. When the electronic device is not powered, the RTC and associated components must be provided with an auxiliary power source. This may be provided, for example, by a coincell battery, embedded in the device. Such an auxiliary power source would be expected to provide power during long periods and in some cases for the lifetime of the electronic device. When the RTC and associated components are powered from such an auxiliary power source, it is therefore particularly important that the RTC etc. have low power consumption, to avoid depleting the auxiliary source too quickly. The power consumption of the RTC and associated components should be in the region of 5 μA or less. Such power consumption levels are usually achievable when the RTC and associated components are situated in the PMIC of the device.
In next generation electronic devices, it is proposed to provide ‘secure’ RTCs. This is to be achieved by implementing the RTC on the microprocessor of the electronic device. This provides greater security for the RTC, and allows provision of additional security features of the RTC. For this partitioning, the associated component of the RTC comprising the oscillator will remain in the PMIC, and a clock line will be required to connect the oscillator and the RTC. The clock line must continuously supply a signal to the RTC, for this to maintain an accurate track of date and time. It is proposed that the clock line signal has a frequency of that of the oscillator, i.e. 32.768 kHz. When this is the case, the clock line will have a power consumption of approximately 2.5 μA. When the RTC, oscillator and clock line are powered from the auxiliary power source, the clock line will use approximately 50% of the desired power consumption of the RTC and associated components. This is obviously not ideal.